(1) Field of the Disclosure
The present invention relates to a ceramic pressure sensor, a method for production thereof, and a transducer which incorporates a ceramic pressure sensor.
(2) Description of Related Art
The use of ceramic pressure sensors integrated in a transducer for measuring the pressure of a fluid is known in various fields of application, such as in the industrial, medical and automotive sectors.
Ceramic sensors combine mechanical robustness and an ability to work in aggressive environments with reliability and stability in terms of performance.
A ceramic membrane, chemically inert, does not require any type of separator and can be in direct contact with many fluids, also with some of the most aggressive ones. Long-term stability and a wide operating range in terms of both pressure and temperature are other important factors which favour the versatility and reliability of ceramic pressure sensors.
These aspects, combined with the simplicity of integration into industrial machinery and process control devices, make both piezoresistive and capacitive ceramic sensors appealing and advantageous compared to other technological solutions, because of both an excellent cost to performance ratio and a wide range of applications.
The transducer is typically provided with a circular conduit suitable for carrying the fluid to be measured inside it and a chamber where the sensor membrane enters into contact with the fluid.
Piezoresistive sensors with a membrane of ceramic material are generally made of alumina, and can be of two types: flat sensors, in which the membrane is fixed on a mechanical support, likewise ceramic, or else dome-shaped sensors, composed of a single ceramic element, in which the thinner central part acts as a membrane and the thicker lateral part act as a mechanical support.
The membrane flexes under the effect of pressure and this flexion can be measured by means of a resistive bridge. The resistors, made of piezoresistive material, are positioned in such a way that, while the membrane flexes, two resistors (belonging to opposite sides of the bridge) lengthen, determining an increase in resistance, and simultaneously the other two are compressed, determining a decrease in resistance. The bridge is unbalanced and the output voltage is proportional to the pressure difference which caused the deformation.
Flat piezoresistive sensors are typically produced by screen printing this resistive bridge onto the membrane, on the side opposite the one in contact with the fluid, and connecting this membrane with a mechanical support, typically obtained by stamping. To ensure sufficient space for deformation of the membrane and define the mobile space of the same, the support is provided with a central recess, typically circular, located in the deformable area of the membrane, where the piezoresistive elements are located, and perforations located outside said recess, on whose walls a conductive material is deposited, and which are electrically connected to the tracks on the membrane after the mechanical connection, achieved for example by soldering or gluing with conductive resins.
In sensors of a capacitive rather than piezoresistive type, the pressure signal is measured by the change in capacitance of a capacitor, one of whose armatures is printed on the membrane and the other on the mechanical support. In some cases a perforation is present on the support, in the recess: if this perforation is open the system will function as a relative or differential sensor (only if the recess is sealed); if it is closed and the recess is sealed it will function as an absolute sensor.
Dome-shaped sensors are easier to produce than flat sensors, since they do not require an electrical connection between the membrane and support.
In particular, dome-shaped piezoresistive sensors, in which the electrical connection is made on the same side as the piezoresistive bridge for measuring pressure, require a manufacturing process with fewer steps than flat ones, and are thus more economical; they represent the solution normally used above all when large quantities are involved.
However, dome-shaped piezoresistive sensors are not suitable for realising either absolute sensors or differential sensors.
Moreover, given their construction, dome-shaped sensors further require that the fluid to be measured enters into the recess. In some applications the presence of a recess can generate problems in their use.
Furthermore, given their construction, dome-shaped sensors can withstand limited values of overpressures, because the membrane under deformation is not opposed or contained and thus ends up breaking if subjected to an excessive deformation; this is also true for flat sensors. Normally, the withstandable overpressure and the breaking stress are very important parameters in the metrological characterization of a sensor. One particular case, of great importance in recent applications, is where the fluid to be measured is a liquid which can break the membrane if it freezes (for example AdBlue, an additive used to reduce nitrogen oxide emissions in diesel engines).
At present, the solution adopted in the automotive sector for this problem uses a dome-shaped sensor, and solutions to the problem of freezing have been patented which are optimized for that type of sensor; they use a disc spring (EP 1252492), a compressible element placed near the sensor (W09831997) or a compressible element placed around a nozzle which conveys the fluid into the recess (WO208078184).